For many applications it is necessary to create and maintain a high vacuum within a vacuum enclosure. For example, in order to maintain components within a cryogenic temperature range it is often necessary to enclose the cryogenically cooled components within a vacuum enclosure in order to minimise the heating of the components. As a result, there is a need for systems and methods for maintaining high vacuums.
As will be readily understood, a high vacuum is any vacuum where the mean free path of residual gases is longer than the size of the vacuum enclosure containing the gases. Generally, a high vacuum is defined as a vacuum having a pressure of about 100 mPa or lower.
In order to create a high vacuum multi-stage pumping is required. Typically, this is achieved by using a combination of a high-vacuum pump and a second-stage vacuum pump. The high-vacuum pump may be a turbo-molecular pump or other similar pump and has an input that is connectable to a vacuum enclosure and an outlet. The outlet of the high-vacuum pump is connected to an input of the second-stage vacuum pump. The second-stage vacuum pump has an outlet that is vented to the surrounding environment. In order to maintain a high vacuum it is necessary for both the high-vacuum pump and the second-stage pump to be continuously operating.
In a typical two-stage pumping system, the inlet of the high-vacuum pump and the vacuum enclosure are maintained at a high vacuum. The high-vacuum pump then acts to compress gas entering the pump such that the pressure of the outlet of the high-vacuum pump is at a higher pressure than the pressure of the inlet and the vacuum enclosure. The outlet of the high-vacuum pump is connected to the inlet of the second-stage vacuum pump. The second-stage vacuum pump is operated to compress gas entering from the high-vacuum pump and has an output that is at a higher pressure than its input. The primary purpose of the second-stage vacuum pump is to ensure that the outlet of the high-vacuum pump is at a low or medium vacuum. This is necessary as many high-vacuum pumps will stall if they are exhausted to atmospheric pressure.
Requiring the continuous operation of two separate vacuum pumps in order to maintain a vacuum can be a problem for some applications. This is because regular maintenance of the vacuum pumps is necessary to keep them in good working order. This can be a particular problem for applications where the vacuum enclosure is located in an inaccessible location. Furthermore, the use of two vacuum pumps can be a problem if the vacuum enclosure is not stationary during operation. One application where there are particular problems is rotary cryostats for superconducting wind turbines. These cryostats rotate during operation and are located in a very inaccessible location, in a nacelle at the top of a wind turbine tower.
Currently it is not possible to use conventional two-stage pumping systems to provide a high vacuum for rotary cryostats for superconducting wind turbines. One reason for this is the very poor conductance down the rotor shaft of such turbines. However, there are also other technical considerations that make the use of conventional, continuously-operating, two-stage pumping systems generally unsuitable. Therefore, current proposals for providing a high vacuum for rotary cryostats for superconducting wind turbines is to use a plurality of getters located in a pre-evacuated high vacuum enclosure. Getters can act to maintain a high vacuum over a limited time period but require re-activation at regular intervals. Re-activating getters in a high vacuum can necessitate re-pressurising the vacuum enclosure in order to access the getters and then, after the getters have been re-activated, pumping the vacuum enclosure to a high vacuum using an external vacuum pump set. Alternatively, non-evaporable getters do not require the vacuum enclosure to re-pressurise but instead require a pumping system to be connected to the vacuum enclosure in order to maintain the vacuum in the vacuum enclosure whilst the getters are re-activated.
In light of the above, there is a need for an improved system and method for providing a high vacuum for vacuum enclosures that are in inaccessible locations and/or are not stationary during operation. Preferably, any such system and/or method should be capable of being used to provide a high vacuum for a rotary cryostat for a superconducting wind turbine or other electrical machine.